Total knee protheses or knees can be divided into two general categories: fixed bearing, where the tibial insert is firmly attached to the baseplate with movement only between the femoral and tibial condylar surface; and mobile bearing, where the insert moves in relation to the baseplate as well as the femoral component. Mobile bearing knees can be further divided into two groups: a rotating platform type, in which the movement between the insert and baseplate is purely internal/external rotation; and the meniscal bearing type, which incorporates gliding in the A/P and M/L directions in addition to rotation.
After total knee replacement, failure is usually caused by loose or worn components. Loose components are caused by excessive loads being transferred to the implant/bone interface that causes interface bonding failure and the implants to loosen. With knee implants, these failures are commonly seen on the tibial side because the implant is susceptible to high shear loading. In a normal intact knee, these shear loads are resisted by soft tissue support and joint congruency.
In order to minimize the transfer of shear stresses to the implant/bone interface with a fixed bearing total knee replacement, the femoral component is allowed to freely slide on the tibia component, using soft tissue to provide constraint. In order to freely slide, these designs require low congruency between the femur and tibia resulting in low contact area. The low contact area causes high contact stresses and contact stresses shorten component life due to material fatigue.
The congruency between the femoral and tibial articulating surfaces of fixed bearing knees must be carefully balanced in order to provide maximum contact area, which lowers the stresses in the polyethylene, yet not be so constrained that normal movement of the femur on the tibia is hampered resulting in high shear stresses.
Mobile bearing knees were developed in an effort to replicate the normal biomechanics of the knee, maximizing the congruency between the tibio-femoral articulation and minimizing shear loading on the tibia. This is accomplished by allowing the tibial insert to be mobile with respect to the baseplate and configured to provide maximum contact area with the femur. These designs are indicated for patients who have adequate collateral ligamentous stability.
The Oxford Knee (Biomet) was an uni-compartmental knee that was the first to use the mobile bearing concept. The plastic "menisci" were totally unconstrained and were prone to dislocation. The LCS Knee (DePuy) was introduced shortly after the Oxford Knee and is the most widely used mobile bearing knee in the world and, having just marked its 20.sup.th year in clinical use, has the longest clinical experience with large numbers. The first LCS design as of the meniscal bearing type with two poly bearings that allowed retention of the ACL and/or PCL. These bearings allowed internal/external rotation and some A/P movement. This design has been fairly successful, although bearing dislocation and fractures have been problems. The second version of the LCS was the rotating only type that sacrificed both cruciate ligaments. This design has also enjoyed a good clinical track record, and because of fewer complications, is used more that the meniscal version of the LCS. A more recent version of the LCS has added A/P gliding.
Other mobile bearing knees in clinical use are: the Rotaglide Knee (Corin), which incorporates a single tibial insert that allows rotation and A/P movement and is approaching ten years of good clinical experience; and the TACK Knee (Link) which is a rotating platform. The more recent SAL (Sulzer) and MBK (Zimmer) Knees are single piece poly designs that allow A/P movement and rotation about a post centered mediolaterally on the baseplate. These two knees have good results at eight and five years, respectively. The Interax ISA (Howmedica) offers A/P movement with asymmetric rotation about a medialized post.
All of the newer designs have added A/P movement in an effort to improve on the rotating platform design or embrace the concept that mobile bearing total knee replacement should replicate, as much as possible, the natural biomechanics and kinematics of the knee while secondarily increasing the contact area of articulating surfaces.